1. Field of the Invention
The present invention relates to a direct current (DC) motor driving circuit; and more particularly to a noiseless driving circuit for DC motor.
2. Description of Related Art
With advancement of electronic technology, a variety of integrated circuits with powerful functions have been created. For example, central processing units (CPU) with billions of transistors are superior to any prior processors embodied in computers. Due to the operations of the tremendous number of transistor, heat resulting there-from is high. In order to maintain/improve service life of integrated circuits and enhance reliability of the circuits, and also more attention is paid on heat dissipation from integrated circuits for improving the reliability and operation speed.
For heat-sink apparatuses of personal computers or CPU therein, a direct current (DC) motor comprising a permanent magnet rotor and one or two stator coils for driving the rotor, have been widely adopted. Due to its low costs and efficiency in heat dissipation, the DC motor has been widely applied in personal computers.
FIG. 1 shows a schematic drawing of a prior art two-coil DC motor. As shown in FIG. 1, the prior art DC motor comprises a voltage regulator 110, a Hall sensor 120, a preamplifier 130, a dynamic offset cancellation circuit 140, a hysteresis comparator 150, a fan lock detection auto-restart 160, a timing control circuit 170, transistors 180 and 190, Zener diodes 191 and 193, and coils 195 and 197.
The voltage regulator 110 is adapted for providing a regulated voltage. The Hall sensor 120 is adapted for sensing the corresponding position of the permanent magnet rotor of the DC motor. Such sensing is amplified by the preamplifier 130 and converted into a square signal varying frequency with the rotational speed of the permanent magnet rotor by the hysteresis comparator 150. Due to the weakness of output signal from the Hall sensor 120, the preamplifier 130 further comprises the dynamic offset cancellation circuit 140 to prevent the voltage offset from the preamplifier 130. The timing control circuit 170, according to the square signal from the hysteresis comparator 150, outputs square-signal control signals C1 and C2 for control of the transistors 180 and 190, respectively. The difference of phases of the square-signal control signals C1 and C2 is 180 degrees. The square-signal control signals C1 and C2 can, therefore, alternatively drive the stator coils 195 and 197 of the DC motor and keep continuous operation of the DC motor.
When the DC motor is accidentally locked, the voltage applied thereto should be removed, lest the DC motor is damaged. The prior art circuit shown in FIG. 1 also applies a fan lock detection auto-restart 160. When the square signal from the hysteresis comparator 150 is not detected, the fan lock detection auto-restart 160 controls the timing control circuit 170 to stop sending the square-signal control signals C1 and C2. The Zener diodes are used to prevent transient pulses resulting from the switch of circuit.
FIG. 2 is a schematic drawing of a prior art one-coil DC motor. As shown in FIG. 2, the prior art DC motor comprises a voltage regulator 210, a Hall sensor 220, a preamplifier 230, a dynamic offset cancellation circuit 240, a hysteresis comparator 250, a fan lock detection auto-restart 260, a timing control circuit 270, transistors 281, 283, 285 and 287, Zener diodes 291 and 293, and a coil 290.
Except of the operations of the timing control circuit 270, and transistors 281, 283, 285 and 287, the operations of other circuits are similar to those in FIG. 1. The timing control circuit 270 outputs square-signal control signals C3, C4, C5 and C6 for controlling the transistors 281, 283, 285 and 287 according to the square signal from the hysteresis comparator 250. The phase angles of the square-signal control signals C3, C4, C5 and C6 cooperate in a manner to alternatively drive the stator coil 290 of the DC motor for continuously operating the DC motor.
The prior art DC motors shown in FIG. 1 and FIG.2, however, use the square-signal control signal to switch the operations of coils. During switching such operations, backing electromotive force (BEMF) may easily cause transient pulses. These transient pulses may generate noises.